To meet demand for high speed wireless access beyond 3G cellular system, E-UTRA (Evolved UMTS Terrestrial Radio Access) has been standardized as LTE (Long Term Evolution) radio access in 3GPP (3rd Generation Partnership Project). Since March 2008, 3GPP has started standardization of IMT-Advanced as LTE-Advanced. The target peak rate of LTE-Advanced is 1 Gbps and 500 Mbps for downlink and uplink, respectively [1]. The higher spectrum efficiency compared to LTE is also required. At the same time, LTE-Advanced should consider backward compatibility with rel.8 LTE as it is developed as the enhancement of LTE.
Downlink CoMP is considered as a key technique to improve the cell-edge throughput and/or to increase system throughput for 3GPP LTE-Advanced [2].
100 and 200 in FIG. 1 show the examples of downlink (DL) transmission without CoMP (w/o CoMP) and with CoMP (w/CoMP), respectively. In FIG. 1, only point 1(101) is evolved in transmission for the UE (user equipment) 104. The UE 104 receives the signal from point 1 but interferences from point 2(102) and point 3(103). However, in 200, three points, i.e., point 1(201), point 2(202), and point 3(203) are involved in coordinated transmission. We use “CoMP point” to indicate the point involved in coordinated transmission, which can be eNode-B (eNB), cell, or other type of node, such as remote antennas connected by optical fiber [3]. A “CoMP UE” is the UE who has more than one CoMP points.
Joint transmission and coordinated scheduling/coordinated beamforming for downlink CoMP have been agreed as candidate schemes for LTE-Advanced [2]. In both cases, multiple CoMP points allocate resource blocks (RBs) with same spectrum positions. In case of joint transmission, multiple CoMP points transmit data simultaneously to CoMP UE 204. As shown in the example of 200, the UE 204 is a CoMP UE. The interference from point 2(202) and point 3(203) are transformed into signals to improve the received signal for the CoMP UE 204. The CoMP gain in terms of the signal-to-interference plus noise ratio (SINR) comes from the diversity gain of all CoMP points, i.e., point 1(201), point 2(202) and point 3(203). However, in case of coordinated scheduling, only CoMP point 1(201) is transmitting data to CoMP UE 204 and the interferences from CoMP point 2(202) and 3(203) are cancelled. The CoMP gain in terms of SINR comes from the interference avoidance of CoMP point 2(202) and 3(203).
How to select CoMP points has great impact on CoMP gain in terms of cell-edge user throughput and average sector throughput. The CoMP gain may be limited when CoMP points are not properly selected to include the potential strong received signal for joint processing or to mitigate the strong interference for coordinated scheduling.
Conventional method to select CoMP point is to use fixed CoMP points to calculate and rank the PF metric (proportional fairness metric) for channel-dependent scheduling. For example, FIG. 2 illustrates the flowchart of conventional method carried out in the scheduler, which is described as follows:
In Step 1, the scheduler receives the UE feedback. The UE feedback includes the UE measured long-term averaged channel information, such as reference signal received power (RSRP) used in [3] or geometry in [4]. RSRP, is determined for a considered cell as the linear average over the power contributions (in [W]) of the resource elements that carry cell-specific reference signals within the considered measurement frequency bandwidth. The geometry is the average signal-to-interference plus noise ratio (SINR) in the presence of shadowing and path loss.
Besides above information, the UE feedback also includes the channel quality indicator (CQI), which is used to indicate the variation of channel state over bandwidth the measured channel state on each RB over RS bandwidth. For example, in LTE, the feedback period for RSRP can be 120˜200 ms, etc. [5]; while, the feedback period for CQI can be 2 ms, 5 ms, 10 ms, etc. [6].
In Step 2, the cell (or other types of points) ranking is carried out for each UE. For each UE, the cells (points) are ranked as follow.Xm,δ≧Xm,1≧Xm,2 . . .whereXm,δ is the uth UE feedback RSRP or geometry of the δth cell in dB, obtained from Step 1.
In Step 3, the candidate CoMP point is selected for each UE and the UE who has more than one candidate CoMP points is regarded as a candidate CoMP UE. For the uth UE, the point with highestXm,δis selected as the serving cell, which is always a CoMP point. Besides the serving cell, the cell with itsXm,δ≧Xm,0−Thresholdis also selected as a candidate CoMP point, where Threshold is pre-defined parameter in dB. For example, with low threshold, only the UEs close to the cell edge may be set as a candidate CoMP UE, who is able to make use of CoMP to improve its throughput.
The Step 4 includes step 306, 307 and 308. In step 306, for each candidate CoMP UE, N points among candidate CoMP points are fixed as CoMP points. Accordingly, a candidate CoMP UE is decided as a CoMP UE when N>1 CoMP points are selected; otherwise, when N=1, a candidate CoMP UE reduces to a non-CoMP UE.
After deciding CoMP points, in step 307, the scheduling metric is calculated and ranked for the preparation of channel-dependent scheduling. For a CoMP UE, the CQI improved by CoMP with fixed N CoMP points is used for metric calculation; while, for the other UE, the CQI without CoMP (N=1) is used.
In step 308, the RBs are allocated for all UEs. The resource allocation is carried out based on the metric ranking. When the UE has N>1 CoMP points, the channel-dependent scheduling allocates the resource blocks with same spectrum positions at each CoMP point.
In the final Step 5, the scheduler informs the scheduling result to the UEs, e.g., the positions and number of allocated RBs.